The present invention relates to acoustic sensors, particularly to an acoustic sensor which can acquire entire acoustic field or acoustic interference patterns, and more particularly to a frustrated total internal reflection acoustic field sensor for use in applications such as ultrasonic transmission tomography.
Ultrasound is used in a number of medical imaging and nondestructive testing modalities. Its advantages include a lack of ionizing radiation and good propagation through dense materials.
A pair of ultrasound modalities that have not been extensively utilized for medical imaging are: (1) acoustic diffraction tomography and (2) acoustic holography. A problem these modalities encounter are due to the lack of a fast way to acquire acoustic fields or acoustic interference patterns.
Due to the radiation risks of x-ray mammography, other approaches for breast cancer screening are being considered. Mammography is currently used for screening women over the age of 40 for breast cancer. It has not been used routinely on younger women because their breast composition is mostly glandular, or radio dense, meaning there is an increased radiation exposure risk as well as a high likelihood of poor image quality. For these younger women, it is calculated that the radiation exposure risk is higher than the potential benefit from the screening. It is anticipated that transmission ultrasound will enable screening of much younger women and complement mammographic screening in women 40 and over.
Ultrasonic transmission tomography holds out the hope of being a discriminatory tool for breast cancer screening that is safe, comfortable, and inexpensive. From its inception, however, this imaging modality has been plagued by the problem of how to quickly and inexpensively obtain the data necessary for the tomographic reconstruction.
Frustrated total internal reflection provides a way to acquire this data. Total internal reflection occurs when light approaches a dielectric interface at or above the critical angle. If there is another interface a short distance away, some light tunnels through the gap between the interfaces, which frustrates the total internal reflection. The amount of light that tunnels is strongly dependent on the size of the gap.
The present invention involves an acoustic field sensor utilizing frustrated internal reflection, which allows the acquisition of the acoustic field over an entire plane all at once and thus solves the prior problems relative to acoustic holography and acoustic diffraction tomography. When an array of sensors, made in accordance with the present invention, is illuminated by a uniform light field, the reflection from the array yields acoustic wave amplitude and phase information which can be picked up electronically or otherwise.